The methods and apparatuses currently used for estimating depth of anesthesia do so mainly by measuring the activity of the cerebral cortex based on EEG (electroencephalography, EEG). For example, Bispectral Index Monitors® (Covidien company) estimate depth of anesthesia by assessing spontaneous activities of the cerebral cortex based on EEG. However, units of EEG signals are microvolts (μV), much lower than electrocardiographic (ECG) signal strength, whose units are millivolt (mV). As a result, detecting EEG signals requires very sophisticated apparatuses, which are usually large, heavy, and are expensive and inconvenient to move or to carry. The costs of special electrodes used in these apparatuses are also high. Therefore, the use of EEG as a method for estimating depth of anesthesia is not economical or efficient. Moreover, anesthetics have different effects on different cerebral regions. Typically, only cortical activities are measured based on EEG signals to determine depth of anesthesia, and other important anesthetic factors may not be evaluated effectively, including motor suppression, analgesic and autonomic activities, which are largely controlled by subcortical regions of the brain. Hence, anesthetic conditions of subjects may not be monitored by only using the methods and apparatuses based on EEG.